1. Field of the Invention
The present invention relates to a measurement method for an optical information medium such as an optical disc.
2. Description of the Related Art
An optical memory technology that employs an optical disc as a high-density and large-capacity information memory medium has been increasingly applied to a digital audio disc, a video disc, a document file disc, and further, a data file and the like. According to the optical memory technology, information is recorded on the optical disc in a form of a minute pit or a minute record mark. Moreover, the information is recorded and reproduced with high accuracy and high reliability by a small focused light beam.
In a Blu-ray disc (BD) which is one of the optical disc, for example, a minute spot is formed in such a manner that a laser beam having a wavelength in a range from 400 nm to 410 nm, specifically, a wavelength of 405 nm is collected by an objective lens having an NA (Numerical Aperture) in a range from 0.84 to 0.86, specifically, an NA of 0.85.
When the pit or the record mark is reproduced by use of the light beam, a reproduction signal is generated. This reproduction signal needs to have a predetermined property in order to ensure stable reproduction thereof in different apparatuses. FIG. 8A shows an example of such a reproduction signal. As an index for measuring the property of the reproduction signal, adopted is a ratio between an amplitude Ipp of an AC component of the reproduced signal and a signal maximum value Itop, i.e., a modulation degree m (m=Ipp/Itop). This modulation degree m needs to be equal to or more than a specific value to ensure compatibility of optical discs among optical disc devices. For this reason, it is possible to ensure the compatibility of the optical disc among optical disc devices by evaluating the optical disc based on a degree of modulation measured by an optical disc evaluation device (measurement optical system).
In the optical disc having a plurality of recording layers, a degree of modulation m is adversely affected by a reflected light (stray light) from other layers than a target reproduction layer. More specifically, in the case where the stray light is contained in the reproduction signal as shown in FIG. 8B, the signal maximum value Itop′ becomes larger by the stray light as compared to the signal maximum value Itop without the stray light from other layers as shown in FIG. 8A. Consequently, a degree of modulation m2 with an effect of a stray light from other layers than the target reproduction layer is expressed by an equation of m2=Ipp/Itop′, which is disadvantageously smaller as compared to the degree of modulation m without the effect of stray light from other layers than the target reproduction layer.
Like the case of the conventional double-layered disc, when an amount of stray light, which is determined by the area of the light receiving section of the measurement optical system, the magnification of the detection system, and the thickness between layers, is smaller than a predetermined amount, it is possible to ensure the compatibility of optical discs among optical disc devices without problem by setting the degree of modulation m to be not smaller than the predetermined level without taking into consideration such conditions as the area of the light receiving section, the magnification of the detection system, etc.
To ensure stable reproduction among different reproduction apparatuses, a value indicative of the difference in reflectance between layers of a multilayered disc needs to be set within a predetermined range.
Specifically, the difference in reflectance between layers needs to be set in the above range to suppress abrupt changes in signal amplitude when a light beam is moved between the layers or the effect of the stray light from other layers. Namely, in the case where a large difference in reflectance exists between layers, the layer of low reflectance is liable to be affected by large stray light from the layer of higher reflectance, which would significantly affect the degree of modulation of a signal. In contrast, like the conventional double-layered disc, in the case where an amount of stray light, which is determined by the area of the light receiving section of the measurement optical system, the magnification of the detection system, and the thickness between layers, is smaller than a predetermined amount, the compatibility of optical discs among optical disc devices can be ensured without problem by setting the reflectance to fall in the predetermined range without taking such conditions as the area of the light receiving section, the magnification of the detection system, etc. into consideration.
Recently, to increase the recording capacity of the optical disc, practical applications of optical discs wherein a recording layer is made up of larger number of layers than two layers, such as a triple-layered recording layer, a quadric-layered recording layer, have been made into consideration. For such high density optical disc made up of three or four layers, it is required to reduce the thickness between the layers. With this structure, an amount of stray light that enters into the light receiving section increases as compared to the case of optical discs of double-layered structure, and therefore, the degree of modulation m, or the reflectance would be largely affected by the degree of modulation m or the reflectance would be significantly affected by factors of the optical system such as an area of the light receiving section, the magnification of the detecting system, and the interlayer thickness of the optical disc. Therefore, when adopting a value indicative of the degree of modulation m or a value indicative of the reflectance set in various measurement optical system, a problem arises in that the compatibility of optical discs cannot be ensured among optical disc devices.
As a solution, it may be considered to set predetermined fixed conditions of an optical system for measuring the degree of modulation m or the difference in reflectance, and the degree of modulation m or the difference in reflectance is measured under the fixed conditions as set. However, this countermeasure requires replacement of all optical systems in measuring machines which currently exist in the worldwide, and consequently such solution is far from reality.